Consumer demands for both higher quality and longer shelf-life foods have stimulated edible film research. The environmental movement has promoted increased concern about reducing disposable packaging amounts and increasing packaging recyclability, further contributing to the recent surge in edible coating and film research. Edible films and coatings are capable of offering solutions to these concerns by regulating the mass transfer of water, oxygen, carbon dioxide, lipid, flavor, and aroma movement in food systems. Edible coatings function by direct adherence to food products; whereas, edible films act as stand-alone sheets of material used as wrappings.
Dried foods, low moisture baked products, and intermediate and high moisture foods all exhibit potential for improvement through the use of edible coatings and films. Dried foods (e.g., dried vegetables and dried meats) and low moisture baked products (e.g., crackers, cookies and cereals) are particularly susceptible to moisture uptake from the atmosphere. Low moisture baked foods are also susceptible to moisture uptake from moist fillings and toppings. Such changes can result in loss of sensory acceptability of the food product, as well as a reduced shelf-life. Many dried and baked products are also susceptible to oxidation, lipid migration and volatile flavor loss.
Intermediate moisture foods, such as raisins and dates, often become unacceptable due to moisture loss over time. Moisture loss is particularly problematic when the moisture transfers into lower moisture components of a food system. For example, raisins can lose moisture to the bran in raisin bran. Nut meats, another intermediate moisture food, are susceptible to lipid oxidation resulting in the development of off flavors.
High moisture food components typically lose moisture to lower moisture components. One classical example of this phenomenon occurs when pizza sauce moisture migrates into the crust during storage, resulting in a soggy crust. Oxidation and flavor loss are also problematic to high moisture food systems. The respiration rates of whole fruits and vegetables often dictate their shelf lives. Minimally processed fruits and vegetables are often subject to unacceptable levels of oxidative browning. Individual food products within the broad food categories discussed above require different barrier properties in order to optimize product quality and shelf-life. Edible films and coatings are capable of solving the barrier problems of these and a variety of other food systems. See, Kester, et al., Food Technol. 40:47-59 (1986) and Krochta, in Advances in Food Engineering, CRC Press, Inc., Boca Raton, Fla. Singh and Wirakartakusumab (Eds.) p. 517-538 (1992).
Edible films and coatings based on water-soluble proteins are typically water-soluble themselves and exhibit excellent oxygen, lipid and flavor barrier properties; however, they are poor moisture barriers. Additionally, proteins act as a cohesive, structural matrix in multicomponent systems to provide films and coatings having good mechanical properties. Lipids, on the other hand, act as good moisture barriers, but poor gas, lipid, and flavor barriers. By combining proteins and lipids in emulsion or bilayer barriers, the advantages of each component can be exploited to form an improved film system. Plasticizer addition improves film mechanical properties.
The properties of composite bilayer films and coatings have been studied in the past. Cohesive bilayer films and coatings are often difficult to form and delamination may occur over time. Furthermore, bilayer film and coating formation often requires the use of solvents or high temperatures, making production more costly and less safe than aqueous emulsion film production. Protein-lipid emulsion film and coating systems can be formed from aqueous solutions and applied to foods at room temperature.
Water-insoluble edible films and coatings offer numerous advantages over water-soluble edible films and coatings for many food product applications. Increasing levels of covalent crosslinking in water-insoluble edible films and coatings result in better barriers to water, oxygen, carbon dioxide, lipids, flavors and aromas in food systems. Film mechanical properties are also improved. Many foods, such as fruits and vegetables, are exposed to water during shipping and handling. In these cases, water-insoluble films and coatings remain intact; whereas, water-soluble films and coatings dissolve and lose their barrier and mechanical properties. Edible films in the form of wraps, such as sandwich bags, also require water-insolubility.
Prior to this invention, water-soluble, protein-based edible films and coatings have been formed from aqueous solutions of proteins (Gennadios, et al., in Edible Coatings and Films to Improve Food Quality, Technomic Publishing Co., Lancaster, Pa., Krochta, Baldwin and Nisperos-Carriedo (Eds.), Chapter 9, (1994)); however, a means to produce water-insoluble films and coatings from aqueous solutions with improved barrier properties had not been discovered. Carmelization and/or maillard browning reactions had been exploited for the formation of improved protein-based oxygen barrier coatings for fruits and vegetables (Musher, U.S. Pat. No. 2,282,801). Protein thiol-disulfide interchange and free thiol oxidation reactions had been studied previously (see Donovan, et al., J. Food Sci. and Technol. 11:87-100 (1987) and Shimada, et al., J. Agric. Food Chem. 37:161-168 (1989)); however, the use of these reactions for the formation of new and improved edible barriers had not been explored. Edible moisture barrier coatings had been formed out of protein-based aqueous emulsions (see, Adams, et al., EP 0 465 801 A1 and Ukai, et al., U.S. Pat. No. 3,997,674); however, methods for the formation of water-insoluble protein-based films and coatings had not been discovered.
Others have studied the interactions between proteins and lipids at interfaces in emulsions and colloidal systems. See, Barfod, et al., in Food Proteins, American Oil Chemists Society, Kinsella and Soucie, eds., (1989) and Le Meste, et al., in Interactions of Food Proteins, American Chemical Society, Washington, D.C., Parris and Barford, eds., (1991). However, regulation of mass transfer as a function of lipid particle size and distribution in films has not been explored.
What is needed is a method for preparing water-insoluble protein-based edible films and coatings from aqueous solutions which exhibit improved barrier and mechanical properties.
Quite surprisingly, the present invention fulfills this and other needs.